Patent GB-708175-A (GB)
Title: Improvements in and relating to indicating scales
Inventor: WIGAN EDMUND RAMSAY
IPC: G01D13/00
Filed: 1950-09-21
Abstract: 708,175. Indicating-apparatus. BRITISH BROADCASTING CORPORATION. Sept. 20, 1951 [Sept. 21, 1950], No. 23217/50. Class 106 (4) Indicating-apparatus, wherein the position of an element is indicated through relative movement between a calibrated scale and another member, bearing one or a plurality of fiducial indices, is associated with range-changing means and comprises means operating automatically in accordance with operation of the range-changing means to move the member relatively to the scale, independently of any movement of the element, into positions where the or an index will be appropriate to the range. The invention may be applied to an R/C oscillator with a circular scale 4 calibrated logarithmically from 20- 200 c/s and read against a fiducial index. Three frequency ranges X1, X10, X100 are controlled by a range switch. A small percentage error constant at all parts of the scale may be corrected for one range by rotation of the scale relative to its mounting shaft but the other ranges may remain in error and may be corrected, according to the invention, by moving the fiducial index automatically relative to the scale into predetermined fixed positions when the range-change switch is operated. As shown in Fig. 1 movement of the index is provided for by using a transparent plate 1 mounted on the shaft 2 of the range-change switch and bearing three fiducial marks 3a, 3b, 3c. The mark 3b may be engnaved on the plate 1 and with its corresponding range in use the scale may be rotated to correct any error. The marks 3a, 3c are pencilled and their positions altered if the frequency scale is in error for their ranges. Parallax is avoided by using powerful local illumination to throw up from the inner face of the plate 1 next to the scale 4 the reflected image of the pencil mark on the outer surface of plate 1. When the pencil marks are correct the plate 1 is removed from the shaft 2 and by a similar optical technique the final index lines are marked on the inner face of the plate 1. As shown in Fig. 2 there is provided one movable mark 6, carried on an arm 7 pivoted at 8 and having its position determined by projections aa, ab, ac on a shaft 9 of the rangechange switch which engage with the remote end of 7. As the range switch is moved these projections, which may be three adjustable screws, cause the index 6 to move relative to the scale 4 to correct for frequency error. As shown in Fig. 3, the arm 7 carries a lens 11 through which the scale 4 is viewed. When the arm 7 is moved, the image of the scale moves relative to the fixed index 12 which may be a blade set on edge in order to avoid parallax. The pivots of arm 7 and scale 4 need not coincide and may be arranged so that a relatively large movement at 9 corresponds to a relatively small movement of the index 6 or 12. The lens 11 may be cylindrical if its movements are small but should be spherical otherwise to avoid distortion. Calibration errors may be corrected as shown in Fig. 4 by a profiled surface 14, which may be a steel ribbon set on edge, fixed to or linked to move with the scale 4, with a roller 17 pressed against the surface 14, the roller being carried by a lever 15 pivoted at 16 and carrying an index 3. The profile 14 may be modified by screws or other means so that the index 3 moves in correspondence to correct the calibration errors of the instrument. An alternative construction is shown in Fig. 6, the shaft of the scale 4 carrying a plate 23 pierced with a number of tapped holes for adjusting screws 19 which press on and appropriately distort tongues cut in a thin flexible plate 18. Engaging the tongues is a roller 20 on an arm 21 pivoted at 22 and carrying a lens 11 or index so that, by suitably adjusting the screws 19, scale calibration errors can be eliminated. The invention may be applied to these arrangements for correcting errors associated with range changing, by arranging for the pivot 16 of Fig. 4 or the pivot 22 of Fig. 6 to be moved a predetermined fixed amount when the range change switch is moved. A number of modifications of index, scale and optical systems ate envisaged.